2. Learning Objective
Having worked through this chapter, the student will
be able to:
Discuss blasting theory
3. Background
In the preceding lectures we have been discussing
topics related to explosives and blasting in general.
In this lecture however, we shall take a brief look into
what happens when the explosive is detonated in the
blasthole in the rock mass: Blasting Theory
4. Introduction
The sudden release of energy and the reaction
products at high pressure by the rapid chemical
reaction in an explosive contained in a blasthole in
rock gives rise to compression waves in the explosive
and in the surrounding rock material.
These compressive waves are called detonation waves
or shock waves, depending on whether they involve
chemical reaction or not.
They play a central role in the functioning of the
explosive and in the fragmentation of the rock mass.
On detonation of an explosive in a rock mass the rock
is affected in 3 principal ways or stages:
5.
6. Stage 1
In this stage, and starting from the point of initiation,
the blasthole expands by the crushing of the walls
immediately surrounding the explosive due to the
development of the high pressure, followed by a
development of radial cracks
7. Stage 2
At this stage compressive stress waves emanate or
radiate in all directions from the blasthole with a
velocity equal to the sonic wave velocity in the rock
mass.
New cracks are developed while old ones are enlarged.
When these compressive stress waves reflect against a
free face in the rock mass, they cause tensile stresses in
the rock mass between the blasthole and the free face.
If the tensile strength of the rock is exceeded in the
process, then the rock breaks in the burden area, which
should be the case in a correctly designed blast,
otherwise the compressive stress waves will just
disappear in the rock mass and the rock does not break.
9. Stage 3
In this stage, large volumes of gases are produced
under very high pressures.
These are dissipated into the cracks formed during the
first and second stages of the process and expand the
cracks.
If the distance between the blasthole and free face is
correctly calculated, the rock mass between the
blasthole and the free face will give way or yield and be
thrown forward, thus fragmenting the rock.
11. Conclusion on Blasting Theory
It must be noted that the explosive reaction in the
blasthole is very fast (in 1000th of a second or
milliseconds) and the effective work done by the
explosive is considered complete when the volume of
the blasthole has been expanded to 10 times its
original volume in approximately 5 milliseconds.
12. Rock Fragmentation
Very often the end purpose of rock blasting is to
produce material that fulfils the objective of the blast.
For example: crushing for mineral beneficiation, road
construction, dam building, erosion control etc., etc.
In the case of mining and construction fragments of
rock should not only be small enough for the loading
and transport equipment,
but should also be small enough to pass through the
crusher opening: a state on which the economic
success of the mining operation hinges, since the
primary crusher becomes a bottleneck, choking the
flow of ore into the beneficiation plant.
13. Rock Fragmentation
Sometimes it is necessary to produce quantities of
large and regularly shaped fragments, as is the case in
dam building or constructing wave breakers.
Even where the main purpose of excavation is to create
an empty space surrounded by rock walls such as in
tunnels, drives, etc., in underground mining the
fragment size is important because it influences the
speed of loading/mucking and the capacity of the
transport equipment.
In all cases therefore the degree of fragmentation
influences the economy of the excavation job.
14. Rock Fragmentation
Basically, 3 main factors control the fragment size
distribution of a blast. These are:
Quantity of explosive used
Its distribution in the rock mass, and
The rock structure
15. Rock Fragmentation
The ideally fragmented rock is that which does not
need further treatment after the primary blast.
The parameters of the subsequent operations are
therefore, the guidelines for deciding on the desired
fragmentation of the formation.
Since the fragment size is of the utmost importance to
the operation all possible efforts should be made to
keep the size down, without making the blast do the
work of the crusher.
